We use a Sears Kenmore that we purchased in the beginning and it still works great, for all the experiments and use it has gotten - a testiment to both Kenmore and the ability of our kilns to protect the microwave from the heat that occurs! Our model is no longer available, however; as of October, 2012, look for Kenmore Elite 1.5 cubic feet model 79153 countertop presently selling for $169, Sears Item # 02079153000 if ordering online or by phone. This model has the required 1200 watts and 10” inside height.
Not sure if still available: Home Depot offers a GE 2.0 cf 1200 watt microwa.ve, model JES2051DNWW (white) and JES2051DNBB (black) for around $159. Sharp model R520LKT 2.0 cf 1200 watt microwave can be acquired at Home Depot online for $149 and at Best Buy, Target, etc.
Note: do not purchase a microwave that says Inverter on the door – it will shut down after running on high for 30 minutes and you don’t want that happening, although you can restart it if you are standing by - it is important not to allow cooling to occur too soon.
Yes but it will take longer to reach smelting temperature inside the crucible. It is also possible that the magnetron may over-heat and thermally shut down. That said, 1100 watts may require 10-15 minutes longer than the normal 60 minutes. Please do not consider using any microwave with less than 1100 watts - it just takes too long, like; forever. Also; if your microwave is used instead of new, you may want to check it for amount of wattage it emits, as the magnetron tends to put out less wattage with age.
Yes. It will take less time - perhaps considerably far less time - to heat the material inside the crucible to liquid state. It would be very important to check on your smelt early and often, to prevent overheating. Smelting time varies between microwaves anyway and is approximate – you will need to experiment at first to determine the best length of time for your equipment and materials.
(We have not experimented ourselves with different microwaves, however, if you will kindly send us your gold concentrates, we will endeavor to do so!)
Glad you asked! A 1200 watt microwave means it takes 1.2 kilowatts of energy to run. Multiply 1.2 x whatever your electric company charges for a kilowatt hour - usually around 11 to 14 cents - and you come out to less than a quarter for an hour of smelting. Bet you thought it was way more than that.
Microwave transparent means that the microwaves travel through the material, rather than bouncing off or heating it. It is important that the kiln material and kiln shelf and crucible are microwave transparent, so that the heat occurs inside the crucible, rather than outside it. If the heat occurs outside of the crucible, it then becomes a radiant heat, rather than a form of plasma arc within the crucible.
Although a microwave with at least 10" clearance is recommended, the different widths of kiln layers enables you to stack your kiln to best fit your microwave. For instance: if you need to make yiour kiln shorter, you can do it one of two ways:
1) Remove the 1" hollow layer beneath the kiln shelf (the kiln shelf is the tile the crucible sits on)
2) If using a shorter fused silica melting crucible (4.5" tall), you can remove one of the 2" hollow layers that surround the crucible and switch it out with a 1" hollow layer (purchased separately).
It just seems to work better than a microwave with a magnetron on the top. Rear is ok, too.
Some metals can reflect microwaves and some metals can absorb a portion of the microwave energy. The microwaves that are emitted from the magnetron are both a magnetic and electrical energy wave, so when any metal becomes charged with electrical energy or reflects the electrical energy, it basically acts just as nature does during an electrical storm. The air is ionized around the metal and the energy that follows these ionized paths of conductivity, discharge, not unlike a lightning bolt. (Lightning bolts are actually a form of plasma containing very high energy and heat.) This heat is contained inside the crucible by the kiln, thereby protecting the microwave from the heat
Yes; anything that is made of cast iron - or graphite - will suffice, even a cast iron muffin tin. However; the conical mold is designed to help the melted metal to form into a single button by taking advantage of gravity and the restricting design of the conical shape, similar to a funnel. A flat mold, if you are mixing flux and smelting ore concentrates, will trap flux underneath instead of on top of the smelt during cooling. Flat molds are great for melting pure metals where flux is used only to coat the crucible beforehand.
Yes, a cast iron mold can be preheated inside an oven but it will not get as hot as it will on top of a gas or electric burner. Remember; the graphite mold needs no preheating - only the cast iron mold needs to be preheated, to help prevent splattering.
No particular temperature. Preheating the mold has two purposes, first to drive any moisture out of the mold. 2300 degree metal and glass and any form of water do not mix. Preheating the mold also helps the flux to flow into the mold more readily. I like to place the mold on a stove top burner and heat it for 10 or 15 minutes; this seems to accomplish both tasks well. Please note: a graphite mold needs no pre-heating.
Give it at least 15 minutes and always wear your protective gear when inverting the mold or working near the pour. Shrapnel can still be ejected even the next day - seen that myself. It's relatively safe to handle when the orange color is gone, the flux turns black and it doesn’t set your gloves on fire when you handle it.
"One shot' crucibles are called so because they are used only one time for fire assaying. For smelting ore concentrates, they should last through several firings if seasoned beforehand and treated properly. With each firing, flux coats the crucible and this actually helps the crucible last longer - that is; as long as the flux does not run over on the outside, or it then causes radiant heating, rather than electrical arc type heating. Check your used crucible for ridges around the interior and, if you find any, be careful of thinning of the wall where the high temperature can then cause it to fail.
Not usually; only clay or fused silica crucibles are microwave transparent. Especially avoid Graphite and Silicon Carbide crucibles, which have carbon in them and are not microwave transparent. If placed inside of the kiln, they would absorb most of the microwave energy and will only allow a small amount of it to pass through them into the interior where the conductive flux and metals are contained. As a result the crucibles would get very hot, however; your heat would be a radiant heat penetrating inward from the crucible, defeating the plasma arcing and heating that takes place directly inside of a clay or fused silica crucible that is so effective in this process.
Fused Silica melting crucibles hold more concentrates and flux but are much more expensive than the throw away “one shot” crucibles, therefore, we recommend you use the 1 shot crucibles for smelting ore concentrates and until you become familiar and comfortable with this process. Fused silica melting crucibles are also best used for melting pure metals, such a nuggets or old jewelry, where less flux is used, as flux tends to pit these crucibles.
No. Leave at least 1” of space to the top of the crucible, so the flux can expand and not overflow the crucible. This is very important - less is better!
Flux is essential for smelting . It stops the reflective process whereby the microwaves merely bounce off of metal and allows the plasma arcing type of heating to occur. It also protects the crucible and refines the gold to a higher degree of purity.
GPK flux is custom mixed and especially designed for the microwave process being used here. It can also be used for propane and other types of smelting equipment.
Litharge: Yes, you can use a lead based flux. In fact, litharge is necessary for doing fire assays, which you can do in the microwave, however, much more care must be taken to avoid the toxic fumes emitted from lead.
The kiln is comprised of the highest quality refractory ceramic fiber insulation available that is microwave transparent (allows microwaves to go through the material) and is suitable for applications up to 1260°C (2300°F). It has low thermal conductivity and high heat storage and is thermal shock resistant for fast thermal cycling with less cracking or spalling. It is factory cut and rigidized to provide for longer life and excellent handling strength and material integrity at high temperatures.
Moisture absorbed from the atmosphere will affect it, so keep your kiln shelf dry and protected from moisture at all times. Also: due to the high temperatures achieved in this microwave process, all kiln shelves will eventually crack due to thermal shock and you may notice a small hairline crack after using it. This is cosmetic only and does not affect the structural integrity of the shelf, as it is well supported by the layer of kiln material beneath it. Even if the shelf were to break completely in two, it would still be safe to use, with caution.
All kiln shelves are not alike. They are composed of different materials targeted for different types of kilns. The kiln shelf that is provided with our microwave kilns is made of high alumina, which is more microwave transparent than other kiln shelf materials. It is also denser and therefore less susceptible to warping and has a higher cone rating of 11, which means it is safe to use up to 2400F degrees.
Shelf materials not to use in your microwave: Silicon carbide shelves have carbon in them. Although rated up to 2730F, carbon is conductive.